The cause of tuberculosis, Mycobacterium tuberculosis is an aerobicbacterium that divides every 16-20 hours. This is extremely slow compared to other bacteria which tend to have division times measured in minutes.

TB is the most common infectious disease in the world. It is more common in undeveloped, tropical countries. In Australia, the incidence of TB is currently 6 per 100,000. Of Australians born in Australia, the rate is 1.8 per 100,000.

Primary tuberculosis is a person's first exposure to M. TB. Assuming the infection was not completely cleared by the immune system (which sometimes happens if the bacterial load was small enough), post primary tuberculosis can occur - this is a reactivation of TB following primary tuberculosis (symptomatic or asymptomatic). In 40% of patients with primary TB, this may take more than 10 years.

Primary tuberculosis is more dangerous than post primary tuberculosis because, if the immune system is compromised or deficient, there is the threat of miliary tuberculosis which infiltrates all the organs of the body and carries a high mortality rate. This is why contact tracing is so important whenever someone is diagnosed with tuberculosis. All his/her close contacts should be screened for TB with a Mantoux test and a chest x-ray.

A persistent respiratory illness that does not respond to regular antibiotics (e.g. penicillin, amoxil) in an otherwise healthy individual is tuberculosis until proven otherwise.

The Mantoux test should be done in all cases of suspected tuberculosis, although the results must be interpreted carefully. Tuberculin units are injected intradermally and read at 48 to 72 hours. An induration of more than 10mm to 10 Mantoux units is considered a positive test. A negative test does not exclude active tuberculosis, especially if the test was done within 6 to 8 weeks of acquiring the infection, if the infection is overwhelming or if the patient is immunocompromised.

Why four drugs? If only one drug is given, what ends up happening is that you end up killing all the bacteria sensitive to that drug and three months later, you get a bunch of bacteria which were resistant. Rifampicin and isoniazid are bactericidal agents that kill the bacteria, pyrazinamide acts well against the intracellular bacteria which are dormant inside macrophages and other cells and ethambutol is a bacteriostatic agent that inhibits bacterial proliferation while the other drugs kill off the TB.

Adverse drug reactions are expected in 20-25% of patients but only 5% all patients will have a severe enough reaction to warrant a change in their drug regimen. Hepatic damage is the most significant of the drug reactions.

Supervised therapy has a cure rate of about 98%.

Prevention

Bacille Calmette-Guerin vaccine
(BCG) immunization gives the receiver between 0-70% resistance to TB. In tropical areas where the incidence of atypical mycobacteria are high (exposure to non-TB mycobacteria give some protection against TB), the effectiveness of BCGs are much lower than in areas where mycobacteria are much less prevalent.